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When talking about body temperature regulation methods of animals both living and extinct, there is more than just “warm or cold-blooded”. Birds and placental mammals are the stereotypical warm-blooded animals, and reptiles are well-known cold-blooded creatures. But then there animals like echidnas and great white sharks that can modulate their average body temperatures by as much as 50 degrees Fahrenheit, making them something between warm and cold-blooded. And now, a new study in Nature Communications by University of California researcher Rob Eagle and co-authors (including me!) indicates some speciess of dinosaur may have been too.

Scientists use the terms endothermy and ectothermy to refer to the primary methods of body temperature regulation in vertebrates. An endotherm is an organism that generates heat internally to maintain a constant body temperature while an ectotherm relies on external or environmental heat sources. To make matters more complicated, there are organisms that have an intermediate state called “mesothermy”—meaning these animals use internal heat to raise their temperature, but not as high as a typical endotherm.

A life reconstruction of an oviraptorid nest by Doyle Trankina and Gerald Grellet-Tinner. (Image via UCLA)

Modern birds are standard endotherms, but what does this mean for extinct bird relatives like dinosaurs? Obtaining the body temperature of an extinct non-avian dinosaur known only from fossils may seem impossible, but with a new isotope technique, it isn’t at all. The hard shells of both dinosaur and bird eggs are made out of a mineral called calcium carbonate, which is comprised of the elements calcium, carbon, and oxygen.

When digested with acid, carbon dioxide (CO2) is released from the eggshell mineral, and the mass of the CO2 produced is measured with a machine called a mass spectrometer. Specific isotopes of carbon and oxygen produce a certain mass of CO2, and the amount of CO2 with that mass can indicate the temperature of mineral formation. Paleontologists call this method “clumped isotope analysis”. As eggs are formed inside the body of a female dinosaur, the temperature of eggshell calcium carbonate formation should be indicative of the animal’s body temperature.

In this study, the eggshells of two types of dinosaurs were analyzed: oviraptorid theropod dinosaurs from Mongolia and titanosaurid sauropods from Argentina. A wide variety of eggshells from modern birds and reptiles were tested to make sure this method was accurately measuring the body temperatures of these animals.

The “paleothermometer” reading from the clumped isotopes indicates something really interesting—these dinosaurs don’t seem to simply be “warm” or “cold” blooded. While the titanosaurid sauropod had a body temperature similar to a large living endotherm at 100.5 degrees Fahrenheit, the measured body temperature of the oviraptorid dinosaurs from Mongolia were considerably lower. The body temperature of a typical endotherm is at least 100.5 degrees Fahrenheit but the body temperature measured from the oviraptorid eggshell is 95 degrees Fahrenheit. This is lower than most modern mammals and birds. Due to the fact these eggshells were carefully analyzed for alteration and were found to preserve original mineral crystals, this low body temperature is likely an accurate signal. This suggests that these dinosaurs may have not been as warm as typical modern endotherms, but also weren’t completely ectothermic.

Where I collected fossils in Mongolia’s Gobi Desert (Photo by Shaena Montanari)

I was involved in this paper as I am a paleontologist interested in using geochemistry to examine fossils and traces from dinosaurs—like eggshells. The oviraptors eggshells in this study were samples that I analyzed for environmental signals for my dissertation research. I collected these eggshells during my time doing field work in Mongolia’s Gobi Desert. When I was walking around the Gobi I noticed these pieces of eggshells were everywhere but they weren’t being used for anything—so why not analyze them for isotopes and learn more about the environment of this habitat 80 million years ago?

While my research focused on just carbon and oxygen isotope ratios, I saw these eggshells were also great targets for this clumped isotope body temperature research, so I was fortunate to collaborate with Rob Eagle and other colleagues. Clumped isotopes are a gateway to understanding shifts in modes of vertebrate metabolism in a way we never thought possible before. Now knowing that some dinosaurs had an “intermediate” state of body heat regulation adds a new layer of understanding to the dinosaur-bird evolutionary transition.